As operational efficiencies in gas turbines have been achieved by higher firing temperatures and with enhanced aerodynamic design of the airfoils, for example, the nozzle stator airfoils, advanced turbines have adopted improved cooling configurations particularly for the trailing edges of the airfoils. It will be appreciated that the hot gases in the gas flow path of turbines may approximate 1700-1800° F. at the first stage airfoils, e.g., the nozzles. With these higher operating temperatures and the advanced aerodynamic designs of the airfoils, improved cooling systems have previously been employed to minimize and slow the rate of damage incurred by the airfoils exposed to these gases. Various configurations of trailing edge cooling systems have previously been proposed and constructed. For example, one such cooling system includes a plurality of radially spaced apertures opening through the trailing edge in combination with radially spaced film cooling holes axially spaced from and adjacent the trailing edge. The film cooling holes are provided along the pressure side, i.e., the hotter side, of the airfoil for film cooling the remaining trailing edge surfaces. Airfoils with such advanced cooling configurations have been provided in gas turbines and have extended the operating life of the turbine nozzles to nearly twice the operating hours of prior cooling configurations. As a consequence, the gas turbines may be operated, e.g., by a utility, for a significantly longer period of time before maintenance is required with the attendant cost advantages not only in terms of turbine efficiency but also turbine downtime.
Turbine airfoils have also been previously repaired by replacing damaged trailing edge portions with a new replacement trailing edge portion. Coupons which constitute a replacement trailing edge portion for a nozzle airfoil have previously been welded to the remaining leading and intermediate sections of an airfoil where the damaged trailing edge sections of the airfoil have been removed. Trailing edge coupons, however, have not heretofore been utilized to significantly extend the operating life of the turbine airfoil as they have lacked the required cooling configurations. The nozzles in current use have only a limited number of hours of operation before the airfoils of those nozzles are so severely damaged as to marginalize the efficiency of the turbine and require repair. For example, many existing turbines have airfoils with a 24,000 hour operation capability. While coupons having similar trailing edge sections as extant in the turbines can be and have been utilized as replacements for the damaged trailing sections, the hours of operational capability of the repaired nozzle airfoils have not been significantly extended and a similar operating life as the original airfoils have been anticipated.
Nozzle airfoils are typically provided in a plurality of nozzle segments arranged in a circumferential array about the turbine axis. Each segment includes inner and outer bands or platforms between which extend one or more nozzle vanes, i.e., airfoils. The segments are typically cast of expensive materials. While it is possible to replace the nozzle segments in their entirety with new nozzles, segments having improved cooling configurations permitting longer term operation, such replacement nozzle segments are prohibitively costly particularly in view of the materials and machining necessary to effect that replacement. Accordingly, there is a need for a trailing edge coupon and a method of repairing turbine airfoils which will extend the operating life of the repaired nozzles beyond the life cycle of the nozzle airfoils originally provided in the turbine.